Ocean sediment data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 174

was to align and adjust all of the records by matching corresponding features,
particularly peaks. They set up an automated correlation algorithm to do this, but
some judgment was needed to determine which features correspond to one another
and to distinguish noise from isotopic features. The result is a curve with d 18 Oon
the vertical axis and an arbitrary scale on the horizontal axis, averaged over the 57
sites for which data were included. Conversion of the scale of the horizontal axis
to a time scale is a critical step in data processing.
While ocean sediments provide data on d 18 O vs. depth, the conversion of
depth to age is dicult for ages > 50,000 years. John Imbrie is perhaps the

world's leading expert on the analysis of ocean sediments. He said:

''Variations in the oxygen isotope content ( d 18 O) of late Quaternary deep-sea
sediments mainly reflect changes in continental ice mass, and hence provide
important information about the timing of past Ice Ages. Because these sedi-
ments cannot yet be dated directly beyond the range of radiocarbon dating
(40,000-50,000 years), ages for the d 18 O record have been generated by matching
the phase of the changes in d 18 O to that of variations in the Earth's precession
and obliquity. Adopting this timescale yields a close correspondence between the
time-varying amplitudes of these orbital variations and those of a wide range of
climate proxies, lending support to the Milankovitch theory that the Earth's
glacial-interglacial cycles are driven by orbital variations'' (Imbrie et al., 1993).

The process of dating sediment cores by comparing with predictions of the
astronomical theory is usually referred to as ''orbital tuning''.
Tuning was proposed initially by Hays et al. (1976). Their tuning procedure
adjusted the time scale by small amounts, within the range of error of radio-
metrically determined dates. M&M referred to such a procedure as ''minimal
tuning''. However, M&M pointed out:

become ever more complex and
diverse. It has been expanded to become the primary mechanism for determining
a timescale, using a large number of adjustable parameters.''

'' ... since that seminal paper, tuning has ...

The major interest in deriving a time scale for ocean sediments and ice cores is
to test theories that purport to explain the occurrence of and transitions between
glacial and interglacial periods, particularly the astronomical theory. Therefore,
the use of tuning would seem to be a form of circular reasoning that assumes the
answer, uses it to adjust the data, and then claims agreement between theory and
experiment. M&M seemed to indicate that they believed that minimal tuning is a
reasonable process, but questioned the more elaborate forms of tuning. The unfor-
tunate thing is that it takes a good deal of effort to ferret out how many time
scales reported in numerous papers are primary and independent and how deeply
tuning has penetrated into the adjustment of time scales.
In order to convert the depth scale to a time scale, a collage of different
techniques is typically applied, some of which involve utilization of previous